ISSN:
1600-5767
Source:
Crystallography Journals Online : IUCR Backfile Archive 1948-2001
Topics:
Geosciences
,
Physics
Notes:
The X-ray diffraction pattern of powdered 1-Cl-4-NO2-benzene was measured at several temperatures in order to characterize the structure of two different phases. For T 〈 282 K, the crystal array is ordered and exhibits P21 symmetry with two molecules in the unit cell; the observed lattice parameters at T = 190 K are a = 5.838 (4), b = 5.218 (3), c = 10.716 (5) Å and β = 96.43 (5)°. A molecular arrangement inside the unit cell which minimizes the crystalline packing energy was calculated and, in combination with the Rietveld method, was used to reproduce the observed diffraction pattern. The refined structure yields an excellent agreement with the experimental results and confirms a qualitative model previously suggested to explain the measured low-frequency Raman spectrum. The high-temperature phase is monoclinic, Z = 2 with a = 3.84 (1), b = 6.80 (1), c = 13.37 (1) Å and β = 97.4 (1)° at T = 290 K. As confirmed by a Rietveld refinement, this phase exhibits an orientationally disordered arrangement in which dipoles of equivalent molecules point along opposite directions. This leads to a statistically centrosymmetrical molecule and enables P21/c symmetry, in agreement with previous studies. The X-ray diffraction pattern also reveals an important increase of the background radiation which, in turn, exhibits a marked modulation of its intensity. On this basis, a simple analytical model has been developed to predict the angular dependence of diffuse scattering due to orientational disorder. The scope and limitations of this model are exhaustively discussed through a detailed comparison with the experimental results.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1107/S0021889899012571
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